Shares of Snapchat’s parent company plunged 9.8 percent Tuesday, to $21.44, as Wall Street skepticism mounts about the financial prospects of the vanishing-photos app.

The shares had been down 13 percent during the session before recovering a bit — only to fall another 1.2 percent in after-hour trading.

Snap’s relatively skimpy revenue and slowing growth — and an admission by 26-year-old co-founder Evan Spiegel that the company will lose money until at least 2019 — are taking their toll, market sources said.

The stock was also taking a hit as investors gained the ability on Tuesday to “short” Snap shares — or bet that they will fall, according to one analyst.

Adding to potential volatility, the company’s underwriters have exercised an over-allotment option to buy an additional 30 million shares, bringing the total IPO to 230 million shares, two sources told Reuters Tuesday.

That means Snap’s bankers would no longer be in a position to stabilize its shares by buying them should they fall below their $17 IPO price. Snap’s opening trade on Thursday was at $24.

The stock — which reportedly has been popular among first-time millennial investors — closed Tuesday at $21.44, off 9.8 percent.

The shares had hit a session low of $20.64 in intraday trades, more than 13 percent under its close a day earlier at $23.77 — which, in turn, was already below the $24 opening price Snap shares got in their splashy debut last Thursday on the New York Stock Exchange.

This week’s selloff has wiped out more than $9 billion of the company’s market capitalization from Friday’s high above $34 billion — a figure which exceeded that of Hewlett-Packard and was just below eBay’s.